This invention relates to expanded or foamed shaped starch products which are made by extrusion under select conditions using starch feed materials having a defined particle size.
Starch, a readily available known biodegradable material, has been used to prepare foamed; film and other shaped products for different purposes including selected packaging applications. Some of these starch products and their uses are described in U.S. Pat. Nos. 5,035,930 and 5,043,196.
Extrusion is a well known technique, widely used in forming plastic materials into various foamed and expanded products. More recently, the extrusion of starch into expanded articles has shown increased interest as noted in the aforementioned '930 and '196 U.S. patents which disclose the extrusion of low density, high amylose starch into packaging materials. Other disclosures of forming starch products by extrusion can be found in U.S. Pat. No. 5,248,702 directed to packaging fillers and in U.S. Pat. No. 5,314,754 involved with the formation of shaped products such as films, rods and sheets. Another disclosure, EP 0 544 234 shows the general process of starch extrusion. However, despite this background of extrusion as well as the known capability of using starch in forming expanded products, the ability to produce different large shaped or structured starch products such as flexible sheets and rigid boards or planks, has not been easily attained.
While much attention has been directed to finding different starch compositions including combinations with various synthetic polymers and additives such as plasticizers, only limited consideration has been given to the nature of the extrusion process itself. Extrusion and expansion of starch not only is different from plastics, but even varies in many respects when large structured products are formed as compared to the formation of smaller products such as cylinders, tubes, rope like products and narrow sheets. In order to produce large shaped products, the need to utilize extruder dies of different shapes and configurations is apparent. However, the ability to make large expanded starch products is complicated by the fact that most of the properties in the smaller formed products have to be maintained or improved while several other properties such as flexibility or rigidity have to be satisfied. The process of forming large structured foam products not only involves a change in the die configuration and size but has to overcome problems or variations caused by increased threshold throughput or feed rates and changes in processing pressure and product cell structure.
Accordingly, there is a need to define a method for producing structured shaped starch products, having the necessary properties such as flexibility or rigidity, and the products produced by this method.